Electron microscope



July 15, 1941. BQERSCH -r 2,249,453 I ELECTRON MICROSCOPE Filed April 29, 1940 Inventors: Han-s Boe1-s|ch, Hans Mah Th e'n- Attorn ey.

Patented July 15, 1941 ELECTRON MICROSCOPE Hans Boersch and Hans Mahl, Berlin-Reinickcm' dorf, Germany, assignors to" General Electric Company, a corporation of New York Application April 29, 1940, Serial No. 322,334

In Germany December 2, 1938 4 Claims.

The present invention relates to an electron microscope in which an electron beam isprojected through the object under investigation to produce a silhouette image of the same on a suitable image-reproducing surface.

- It is an object of the invention to provide an improved microscope of simple construction which is characterized by high magnification and resolving power. In the attainment of this object, an important feature of the invention consists in the provision of means by which the elecelectron source which may be alternatively em ployedin the apparatus of Fig. 1. 7 Referring particularly to Fig. 1, there is shown an electron microscope comprising a plurality of electrode elements which are assumed to be enclosed in an evacuated container (indicated by dotted line l0). An object to be investigated by the microscope is shown as being supported at I l in a suitable frame l2. At the right of the object there is positioned an image-reproducing surface, for example, a fluorescent screen on which it is proposed to silhouette the object by means of an electron beam which is caused to pass through the-object. 1

In accordance with our present invention, the electron beam referred to-is produced by the combination of a cold cathode l6 and an accelerating electrodeor anode l1. The cathode I6 is of pointed configuration and isso positioned with respect to the electrode l1 that field emission eifects causeelectrons to proceed from the tip of the cathode toward the'screen l4. As a'result of the particular cathode shape chosen, the resulting electron beam proceeds essentially from a point source. If desired, special electrode agencies" of known character may be utilized for 7 increasing the field intensity in the vicinity of the cathode tip. For example, one may employ in this connection a foil or a grid, in some cases so formed as to produce a lens effect.

It is desired to cause the electrons which are generated by the cathode [6, to. pass through a second region, .also of point: dimensions, which is extremely close to the object H, such a point being indicated at l9. When .the condition specified is attained, the subsequent divergence of the electron beam as it proceeds from the point 19 toward the screen I4v results; in the projection on the screen of a magnified image of the object under inspection. The amount .of..magnification obtainable in this way is given by the ratio of the distance between the point l9 andlthe object 'l l to the distance between the point l9 and the screen It.

In order to cause the electrons emittedfrom the cathode 16 to passthrough the point 'l9,.an electron lens is interposed between the cathode and the object supporting frame l2. This lens maybe of either magnetic or electrostatic character and is illustrated by way of example in the drawingas comprising a'pair of successively arranged cylinders 22 and 23. By .the use of these cylinders, appropriately biased, the courseof the electronbeam may be controlled as indicated by the dottedrays 24. If desired,-an imagefield stop diaphragm 25 may be provided in the region be tween theextremity of the cylinder 23- and the image ll-to cut out the marginal regions of the electron beam and thereby to eliminate any lens faults which-would prevent the realization of a sharp point at I-9.'- The stop 25 further limits the angle of aperture of the electronbundle. Accordingly, this angle can be sofdimensioned that it is smaller than the'smallest Bragg angle'of deflection, whereby blurring of the image by deflected electron is avoided.

r A microscope constructed'in accordance with the present invention has unusual simplicity as a result of the absence of lensesbetween the object I IV and .the luminous screen M. U Even the single lens which is. employed between thecathode and the object can be, of simple ,desigmsince the use of a point source of electron emissiongreatly simplifies the problem of obtaininga point concentration of the beam at, Hi; In this connection. the use of .a pointed coldcathode. from which electrons aregenerated by field'emission,

as in the arrangement (illustrated, is especially advantageous since it results in the development of an electron beam which is characterized both 7 by high intensity and by a high degree of concentratiom;

Instead of the cathode illustrated in Fig. 1, it is possible to use a cathode of more usual construction. Thus, in the alternative arrangement which is shown in Fig. 2, use is made of a cathode 30 which has a flat emitting surface 3|. In order that the actual source of the electron beam used in the object-irradiating process shall be of point dimensions, a virtual cathode can be formed by the use of a perforated diaphragm 32 having therein a minute perforation 33. For the purposes of the present invention, the perforation 33 is subjected to the impingement of electrons projected from the cathode 30 and is thereby caused to become a secondary source of electrons whose dimensions are determined by the dimensions of the opening 33. This opening may readily be made with a diameter as small as cm. Thereafter, by projecting the electron beam which originates from this perforation through an electron lens having a 1000-fold reduction, one may obtain at point i9 (Fig. 1) a beam having a diameter of 10- cm.; that is, of molecular dimensions. Consequently, distances of this order of magnitude can be resolved with the arrangement illustrated. Even if thediaphragm opening 33 has a diameter-of 10- cm. and the lens system only brings about a reduction of 1/100, the diameter of the beam at I9 still becomes 10* cm., which compares favorably with the limit of resolution of the photo-microscope. It will be understood that in using the electrongenerating combination of Fig. 2, the electrons from the cathode 30 may be concentrated on the perforation 33 by the use of any appropriate means such as one or more concentrating electrodes.

It has already been pointed out that the degree of magnification which is possible with the arrangement described depends upon the closeness of the point [9 and the object II. It should be further noted in this connection that this rule applies irrespective ofwhether thepoint l9 lies in front of or behind the plane of the object.

The various electrode elements may be energized in any desired-fashion. In the arrangement shown, they are variously connected to a series of batteries 35, 36 and 31. It is obviously possible, however, to energize all of the elements from a common voltage source by means of a voltage divider so' that the operation of the apparatus is free from objectionable effects due to voltage fluctuations.

What we claim as new and desire to secure by Letters Patent of the United States is: 1'. An apparatus for investigating objects of microscopic dimensions by electron optical means comprising means for supporting an object to be investigated, means .providing an imagereproducing surface, S and means for projecting a beam of electrons through the object toward the surface, said last-named means including a cathode which is materially spaced from the object-supporting means and which has an active emitting surface of essentially point dimensions, and an electron lens system for causing electrons proceeding from said cathode; to pass through a region of essentially point dimensions in close proximity to the plane of the object, the space between the object and the image-reproducing surface being substantially devoid of electric and magnetic fields so that electronsare free to proceed along substantially straight divergent paths from the said region to the said surface.

2. Apparatus for investigating objects of microscopic dimensions by electron optical means comprising means for supporting an object to be investigated, an image-reproducing surface spaced from the object-supporting means, and means for projecting a beam of electrons through the object toward the said surface, said lastnamed means including an electron source which is materially spaced from the object-supporting means and which has an active emitting surface of dimensions less than 0.01 centimeter in a direction transverse to the path of the electron beam, and an electron lens system for causing electrons proceeding from said source to pass through a region of essentially point dimensions in close proximity to the plane of the object, the space between the object and the imagereproducing surface being devoid of magnetic and electric fields of converging effect whereby the natural divergence of the electrons in said space causes a magnified replica of the object to be produced on the said image-reproducing surface. I V

3. Apparatus for investigating objects of microscopic dimensions by electron, optical means comprising means for supporting an object to be investigated, an image-reproducing surface spaced from the object-supporting means, and means for projecting a beam of electrons through the object toward the said surface, said lastnamed means comprising a cathode body which is tapered to a point-like tip directed toward the object-supporting means, means for causing electron emission fromthe cathode to occur mainly from the said tip, and an electron lens system for causing electrons proceeding from said'cathode to pass through a region of point dimensions in close proximity to the plane of the object, the space between the object and the image-reproducing surface being substantially devoid of electrio and magnetic fields whereby electrons are free to follow straight divergent paths from said region of point dimensions to the image-reproducing surface.

4. Apparatus for investigating objects of microscopic dimensions by electron optical means comprising means for supporting an object to be investigated, an image-reproducing surface materially spaced from the object-supporting means, and means for projecting a beam of electrons through the object toward the imagereproducing surface, said last-named means comprising a source of electrons, a diaphragm interposed between the source of electrons and the object and having therein a perforation of diameter less than 0.01 centimeter, means for causing electrons emitted from said source to irradiate the perforated portion of the diaphragm so as to cause secondary electrons to be emitted from the boundaries of the perforation, and an electron lens system interposed between said diaphragm'and the said object tocause the'said secondary electrons to pass through a region of point dimensions in close proximity to the plane of the object, the space between the object and the image-reproducing surface being devoid of magnetic and electric fields of converging effect whereby the natural divergence of the electrons in said space causes a magnified replica of the object to be produced on the said image-reproducing surface. i I HANS BOERSCH.

HANS MAHL. 

